The art of coal treatment to upgrade coal and provide alternative fuels, particularly liquid fuels to replace petroleum-derived liquid fuels, was first studied intensively in Germany in the 1920's. Research in the technology of coal upgrading has continued up to the present time, and was particularly active during the worldwide oil shortages of the 1970's.
Techniques for recovering more-easily utilized fuels from raw coal are generally known as coal liquefaction. Coal liquefaction can employ a wide variety of non-anthracitic substrates, particularly bituminous, sub-bituminous and lignitic coals. Other organic materials, e.g., peat can also be used.
Coal liquefaction processes broadly include both thermal (non-catalytic) and catalytic procedures. In thermal processes, heat is used to liquefy the coal without addition of extraneous catalytic materials. In thermal coal liquefaction processes, however, minerals, especially iron-bearing species, naturally found in the coal substrate may function as catalysts for the process. Both catalytic and non-catalytic coal liquefaction processes can be performed in a variety of reactors, including slurry phase reactors and fluidized bed reactors.
Coal liquefaction processes attempt to bring about cleavage of weak heteroatom to carbon and stronger carbon to carbon linkages in the coal structure. In the context of coal liquefaction, heteroatoms include nitrogen, oxygen and sulfur, bonded in any fashion to carbon of coal. The intermediate free radicals, resulting from cleavage of carbon-heteroatom and carbon-carbon bonds, are hydrogenated during liquefaction to prevent polymerization of the thus-produced free radicals to high molecular weight structures.
Although hydrogen performs the necessary function of hydrogenation in coal liquefaction, it has been found that introduction of hydrogen by a hydrogen donor solvent is preferable to use of gaseous hydrogen alone. Hydrogen donor solvents must dissolve the products from coal liquefaction and must be capable of reversible hydrogenation and dehydrogenation. The donor solvent therefore functions as a hydrogen carrier, upon which hydrogen is loaded and introduced into the reaction mixture. Hydrogenated donor solvent then transfers hydrogen to free radicals generated during coal liquefaction and the hydrogen-depleted solvent is separated from the products and is rehydrogenated before recycling to the coal liquefaction reaction.
Corcoran et al. (U.S. Pat. No. 4,388,171) have disclosed extraction of coal under supercritical conditions with a solvent mixture, characterized by molecular diameter with respect to the pore size of the coal being treated. The use of a mixture of polar and non-polar solvents is proposed.
Zinniel et al., in U.S. Pat. No. 3,637,639, have proposed extraction of resinous materials from coal by a series of solvent-extraction and solvent-separation steps, particularly using a mixture of hexane and chloroform.
Stiller et al. have proposed, in U.S. Pat. No. 4,272,356, extraction of coal with highly polar solvents, such as dimethyl acetamide and hexamethyl phosphoramide. A deficiency of the process is the difficulty in recovering solvent for recycling.
Sze et al., in U.S. Pat. No. 4,028,221, have proposed pretreating sub-bituminous or lignitic coal, intended for hydroliquefaction or solvent refining, by removing at least 10% of organic oxygen in the coal being used. The organic oxygen is removed by soaking the coal at 288.degree.-400.degree. C. in a pasting solvent derived from the hydroliquefaction process. A stated advantage of removing organic oxygen from the coal is reduction in the amount of hydrogen required for the liquefaction reaction.
Bay (U.S. Pat. No. 4,089,658) has disclosed extracting coal with a mixture of water, carbon tetrachloride and one or more additional organic solvents. The extract is used as a fuel additive.
Heating a wet coal slurry at 100.degree.-350.degree. C. in the presence of a solvent has been recited by Komiyama et al., in U.S. Pat. No. 4,344,837, as a way of dehydrating wet coal.
Wilson et al. (U.S. Pat. No. 3,692,662), have recited a two-step process for hydroliquefaction, in which the coal is first treated in a hydrogen-donor solvent slurry at temperatures of 260.degree.-370.degree. C. and agitated until the viscosity of the slurry indicates that dispersion is complete. The reference represents that use of a thus-formed dispersion for hydroliquefaction gives higher yields of liquefaction products. It will be appreciated that the temperatures used are above the lower limit, at which depolymerization of coal occurs.
The use of a hydrogen donor solvent, in the vapor phase, to swell coal particles before liquefaction has been proposed by Long et al. (U.S. Pat. No. 4,250,014). The swollen coal particles are then subjected to liquefaction conditions, in the presence of a liquid-phase solvent. Pretreatment with a hydrogen donor solvent, e.g. tetralin, is reported to have no effect on coal conversion, but treatment with tetrahydroquinoline is cited as giving significantly higher coal conversions.
Prior art references, teaching solvent pretreatment of coal before solvent refining accordingly teach, in many cases, pretreatment at temperatures as high as, or higher than, those used for hydroliquefaction. The processes may be commercially unacceptable because of requirements for high temperatures or pressures or because, as a result of high temperatures or pressures employed, the coal residue takes on a refractory character, and does not readily undergo further processing with a hydrogen donor solvent.
It is therefore an object of this invention to provide a method for pretreating feed for hydroliquefaction reactions, in which coal is subjected to relatively low-temperature extraction with a binary solvent mixture, to produce feed which gives enhanced oil yields during subsequent hydroliquefaction.